Crosslinkable composition for coating substrates, coating process employing this composition, and substrates thus coated

ABSTRACT

A curable composition comprising (A) a modified copolymer which is a partial ester of a styrene-carboxylic anhydride copolymer, a hydroxyalkyl (meth)acrylate and, if required, a monovalent alcohol; (B) at least one diluent for the modified copolymer; (C) at least one multifunctional epoxy; and (D) a photoinitiator or a system capable of initiating photopolymerisation; and being characterised in that it further includes (E) at least one catalyst for the reaction between carboxylic acid functions and epoxy functions. The composition is useful for coating substrates by heat and radiation curing.

The present invention relates to a crosslinkable composition which canbe applied for coating substrates, and to a coating process employingthis composition and to the corresponding coated substrates.

The substrates in question are substrates of all kinds, both porous,like wood, and nonporous, like glass, metals or plastics, and bothplanar in shape and comprising relief or hollow regions or, moregenerally, of complex shapes giving rise to regions, called shadowregions, which cannot be reached by irradiation using UV lamps orelectron beams under which the components to be coated are travelling.This is, for example, the case with wood exhibiting pores which open atthe surface or complex mechanical items like engine components.

The technical requirements of the protective varnishes and coatingsaddressed by the present invention are very diverse and sometimes highlyrestrictive; they are generally the following:

excellent adhesion to metals such as aluminium and steel and to plasticssuch as polycarbonate;

excellent mechanical properties, in particular of flexibility, modulusand tensile strength;

very good substrate wetting properties;

good dispersibility of organic and inorganic pigments;

good adhesion as interface layer;

excellent adhesion to substrates with shadow regions, in particularporous substrates such as wood, and obtaining a complete crosslinking,it being known that, in the contrary case and with porous substrates,the remaining monomers can exude to the surface of the coating in thecourse of time, in particular with the disadvantage of producing anunpleasant odour;

excellent adhesion to all types of substrates of complex shapes, even inthe shadow regions.

Until now a high level of these requirements has never been satisfiedsimply, economically and with respect for the environment and, inparticular, on coatings applied to substrates comprising shadow regions,a hard-ness of the portion of the coating of the shadow region has notbeen obtained, it being possible, as indicated, for this shadow regionto comprise thick regions in which the crosslinking under the effect ofan irradiation is found to be difficult.

Patent Application JP-A-62/158710 discloses a photocrosslinkablecomposition which can be used especially for the manufacture of inks,including:

(a) 100 parts by weight of a mixture consisting of

(1) 40 to 80% by weight of a modified copolymer obtained by ring openingof 10 to 100% of the anhydride rings of a maleic anhydride-unsaturatedhydrocarbon copolymer using a hydroxyalkylene (meth)acrylate; and

(2) 20 to 60% by weight of a modified copolymer prepared by ring openingof 50 to 100% of the anhydride rings of a maleic anhydride-unsaturatedhydrocarbon copolymer using a saturated compound containing a hydroxylgroup;

(b) from 5 to 40 parts by weight of a resin obtained by ring opening of0 to 70% of the epoxy rings of an epoxidized phenolic resin using(meth)acrylic acid;

(c) from 5 to 100 parts by weight of photopolymerizable monomers; and

(d) from 0.5 to 15 parts by weight of photoinitiators.

Patent Application JP-A-63/205649 discloses a photosensitive liquidcomposition which can be used as a soldering mask for printed circuits,including:

100 parts by weight of the mixture of an ester (a) obtained by reactionof a copolymer formed by at least 10 mol % of maleic anhydride and atmost 90 mol % of monomer which is polymerizable by a radical route with0.05 to 0.95 moles, per mole of anhydride, of alcohol and/or ofmonovalent epoxide which can be polymerized by a radical route and of anepoxide (b) chosen from bisphenol diglycidyl ether and epoxy resins ofcresol-novolac or phenol-novolac type;

from 10 to 200 parts by weight of a compound which can be polymerized bya radical route, containing no epoxy group; and

an organic solvent.

Hydroxyalkyl (meth)acrylates are mentioned among the monovalent alcoholswhich can form part of the composition of the ester (a). Glycidyl(meth)acrylate, allyl glycidyl ether and cyclohexenevinyl monoepoxideare mentioned among the monovalent epoxides which can form part of thecomposition of the ester (a).

Already known from Patent U.S. Pat. No. 5,114,830 is a composition,crosslinkable under the effect of a radiation, including from 15 to 40%by weight of a partial ester of a hydroxyalkyl (meth)acrylate and of astyrene-maleic anhydride copolymer, from 2 to 15% of a multi-functional(meth)acrylate monomer, from 3 to 12% of a photoinitiator and from 10 to35% by weight of a multi-functional epoxide. This composition, intendedfor crosslinked soldering masks for printed circuits, makes it possibleto solve the technical problems previously attributed to the postcuring(thermal crosslinking) stage which follows the crosslinking usingirradiation. In fact, during the curing stage (at a temperature of 130°to 170° C.), a substantial proportion of the hydroxyalkyl (meth)acrylategroups dissociates from the polymer, thus regenerating maleic anhydridegroups which can react with the multifunctional epoxide to perform a newand more stable crosslinking of the photopolymer. In all probabilitythis involves the formation of an interpenetrating network of, on theone hand, poly(meth)acrylate and, on the other hand, of epoxide-graftedstyrene-maleic anhydride copolymer.

Patent U.S. Pat. No. 5,296,334 discloses a photocrosslinkablecomposition including (a) 5 to 50% by weight of an esterifiedstyrene-maleic anhydride copolymer which has less than 15% of freeanhydride, in which at least 50% of the anhydride groups are esterifiedwith a hydroxyalkyl (meth)acrylate and at least 0.1% of the anhydridegroups are esterified with a monovalent alcohol, (b) 5 to 20% by weightof a multifunctional (meth)acrylate monomer, (c) 1 to 30% by weight of amultifunctional epoxide and (d) 1 to 15% by weight of a photoinitiator.This composition provides a soldering mask for printed circuits after aprecure at a temperature of 49° to 77° C., followed by an exposure tohigh-energy radiation, followed by a postcure at a temperature of 121°to 160° C. for 5 minutes to 2 hours. In this technology the exposure toradiation results in a reaction between the (meth)acrylate groups of thecopolymer (a) and the (meth)acrylate groups of the monomer (b).

It has now been found that, by reacting in a different manner theingredients of the compositions of Patents U.S. Pat. Nos. 5,114,830 and5,296,334 with, in addition, an appropriate catalyst, it is possible toform protective varnishes and coatings which can be employed on a widevariety of substrates in applications in which the technicalrequirements, indicated above, are different from those (heatresistance, chemical resistance to solvents and to the acids employed insoldering, adhesion to copper, stability in high humidity conditions) ofthe soldering masks for printed circuits.

A first subject of the present invention consists of a crosslinkablecomposition including:

(A) a modified copolymer which is a partial ester of astyrene-carboxylic anhydride copolymer, of a hydroxyalkyl (meth)acrylateand, where appropriate, of a monovalent alcohol;

(B) at least one diluent for the said modified copolymer;

(C) at least one multifunctional epoxide;

(D) a photoinitiator or a system capable of initiatingphotopolymerization, characterized in that it additionally includes:

(E) at least one catalyst of the reaction between carboxylic acidfunctional groups and epoxy functional groups.

(A) Modified Copolymer

The styrene-carboxylic anhydride base copolymer which can be used forforming the component (A) of the crosslinkable composition according tothe invention has, preferably, a number-average molecular weight ofapproximately between 500 and 4000; it is generally of the alternatingtype and it preferably has a molar ratio of styrene to the carboxylicanhydride of approximately between 1 and 4.

Examples of unsaturated carboxylic anhydrides forming part of thecomposition of the styrene-carboxylic acid base copolymer which may bementioned are the anhydrides of maleic, itaconic and citraconic acids,in particular maleic anhydride.

The hydroxyalkyl (meth) acrylate which can be used for modifying thestyrene-carboxylic anhydride base copolymer is, preferably, an acrylateor methacrylate in which the hydrocarbon part of the hydroxyalkyl grouphas approximately from 2 to 12 carbon atoms, such as the hydroxyethyl,hydroxypropyl, hydroxybutyl or tetrapropylene glycol (meth)acrylate.

As examples of monovalent alcohols which can, where appropriate, be usedfor modifying the styrene-carboxylic anhydride base copolymer there maybe mentioned short-chain (from 1 to 6 carbon atoms) aliphatic alcoholssuch as methanol, ethanol, propanol, butanol and 1-methoxy-2-propanol,and long-chain (approximately 7 to 22 carbon atoms) aliphatic alcohols,such as stearyl alcohol.

For good effectiveness of the crosslinkable compositions according tothe invention it is preferable that the molar ratio of the hydroxyalkyl(meth)acrylate to the carboxylic anhydride functional groups present inthe styrene-carboxylic anhydride copolymer should not exceed 1.

(B) Diluent for the Modified Copolymer

The diluent for the modified copolymer present in the crosslinkablecomposition according to the invention may be either an unreactivediluent of the light alcohol type (like methoxypropanol) or else asaturated ester (like butyl acetate), or a reactive diluent such as amultifunctional (meth)acrylate, and more particularly a polyolpoly(meth)acrylate, like ethylene glycol, propylene glycol,1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol,1,4-cyclohexanediol, 1,4-cyclohexanedimethanol,2,2,4-trimethyl-1,3-pentanediol, 2-ethyl-2-methyl-1,3-propanediol,2,2-diethyl-1,3-propanediol, diethylene glycol, dipropylene glycol,triethylene glycol, tripropylene glycol, tetraethylene glycol,tetrapropylene glycol, trimethylolethane, trimethylolpropane, glyceroland pentaerythritol diacrylates and dimethacrylates, trimethylolethane,trimethylolpropane, glycerol and pentaerythritol triacrylates andtrimethacrylates, pentaerythritol tetraacrylates and tetramethacrylates,dipentaerythritol di(meth)acrylates to hexa(meth)acrylates,poly(meth)acrylates of mono- or polyethoxylated or mono- orpolypropoxylated polyols, such as triethoxylated trimethylolpropane andtripropoxylated trimethylolpropane triacrylate and trimethacrylate,tripropoxylated glycerol triacrylate and trimethacrylate,tetraethoxylated pentaerythritol triacrylate, trimethacrylate,tetraacrylate and tetramethacrylate, and mixtures thereof in anyproportions. Among these, preference is given to those which have theeffect of lowering the viscosity of the composition, such as especiallyethoxylated trimethylolpropane triacrylate and polyethylene glycoldiacrylates.

The proportion of diluent in the crosslinkable composition according tothe invention is approximately between 5% and 200% by weight, preferablyapproximately between 25 and 100% by weight, relative to the sum of thecomponents (A)+(C), according to the viscosity desired for the finalcomposition and the nature of the diluent and of the multifunctionalepoxide which are employed.

(C) Multifunctional Epoxide

The multifunctional epoxide employed in the crosslinkable compositionaccording to the invention has at least two epoxy functional groups.More particularly it is possible to mention diepoxidized resins:

of cycloaliphatic type like, for example, those containing acyclohexene, tricyclodecene or cyclopentene oxide group,3,4-epoxycyclohexylmethyl-3', 4'-epoxycyclohexane,3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate,vinylcyclohexene dioxide, 2,2-bis(3,4-epoxycyclohexyl)propane,bis-(2,3-epoxycyclopentyl) ether,2-(3,4-epoxy)cyclohexane-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane andbis(3,4-epoxy-6-methylcyclohexyl) adipate;

of aliphatic type like, for example, butadiene diepoxide and diepoxidesof polyglycols of general formula: ##STR1## in which R is chosen fromthe hydrogen atom and the methyl radical and n is an integer rangingapproximately from 1 to 15; and

of aromatic type like, for example, bisphenol A diglycidyl ether,bisphenol F diglycidyl ether and ethoxylated bisphenol A diglycidylether.

To make use of the invention it is also possible to employ aliphatictriepoxidized resins like, for example, glycerol triglycidyl ether andtrimethylolpropane triglycidyl ether and tetraepoxidized resins like,for example, tetraglycidylbisaminomethylcyclohexane, and multiepoxidizedresins like, for example, epoxy novolacs.

For good effectiveness of the crosslinkable composition according to theinvention it is preferable that the molar ratio of the carboxylic acidfunctional groups of the modified, that is to say partially esterified,copolymer (A) to the epoxy functional groups of the multifunctionalepoxide (C) should be between 0.3 and 2.

(D) Photoinitiator or System Capable of Initiating thePhotopolymerization

The photoinitiator employed in the crosslinkable composition accordingto the invention may be any compound capable of generating free radicalsunder the effect of ultraviolet radiation.

Examples which may be mentioned are:

α-diketones like benzil and diacetyl;

acyloins like benzoin;

acyloin ethers like benzoin methyl ether, benzoin ethyl ether, benzoinisopropyl ether and benzoin isobutyl ether;

thioxanthones like thioxanthone, 2,4-diethylthioxanthone,thioxanthone-1-sulphonic or isopropylthioxanthone-4-sulphonic acid,isopropylthioxanthone and 2-chlorothioxanthone;

benzophenones like benzophenone, 4,4-bis(dimethylamino)benzophenone,4,4'-bis(diethylamino)benzophenone or 4,4'-diethylaminobenzophenone,Michler's ketone;

propiophenones like 2-hydroxy-2-methylpropiophenone or4'-isopropyl-2-hydroxy-2-methylpropiophenone;

acetophenones like acetophenone, p-dimethylaminoacetophenone,α,α'-dimethoxyacetoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone,p-methoxyacetophenone, 2-methyl-4-(methylthio)phenyl!-2-morpholino-1-propanone,2,2-diethoxyacetophenone, 4'-phenoxy-2,2-dichloroacetophenone,2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl) -1-butanone,2,2-dimethoxy-2-phenylacetophenone or2-hydroxy-2-methyl-1-phenylpropanone;

quinones like anthraquinone, 2-ethylanthraquinone,

2-chloroanthraquinone or 1,4-naphthoquinone;

alpha-hydroxyaryl ketones like 1-hydroxycyclohexyl

phenyl ketone;

halogenated compounds like phenacyl chloride, tribromomethyl phenylsulphone or tris(trichloromethyl)-s-triazine;

peroxides like di-t-butyl peroxide; and

other compounds, like benzil dimethyl ketal, isoamylN,N-dimethylaminobenzoate, ethyl N,N-dimethylaminobenzoate, benzoinbenzoate, 2-hydroxy-2-methyl-1-phenylpropanone or α-acyloxime ester.

These compounds can be employed either individually as a photoinitiatoror in the form of a mixture of at least two of them, as aphotoinitiating system. Furthermore, at least one photoactivating agentmay be associated with the intitiator or the photoinitiating system.

The proportion of photoinitiator or of photoinitiating system in thecrosslinkable composition according to the invention is, for example,approximately between 2 and 15%, preferably approximately between 3 and9%, relative to the weight of the sum of the components (A)+(C).

(E) Catalyst According to the Invention

The catalyst of the reaction between the carboxylic acid functionalgroups of the modified copolymer and the epoxy functional groups of themultifunctional epoxide may be chosen especially from tertiary amines,betaines, organic onium acetates, triphenylphosphite chromium acetateand quaternary salts of tertiary amines.

The proportion of the catalyst in the crosslinkable compositionaccording to the invention is, for example, approximately between 1 and10% by weight, preferably approximately between 1.5 and 7% by weight,relative to the sum of the components (A)+(C).

Other Additives

The crosslinkable composition according to the invention mayadditionally include, with a view to certain specific applications,additives such as:

inorganic pigments like titanium dioxide, iron oxides, carbon black, ororganic ones like copper phthalocyanine blue, β-naphthol red, azoyellows and dioxazinc violet, it being possible for these pigments to beemployed in a proportion of 0.5 to 40% by weight relative to the sum ofthe components (A)+(C);

polymer waxes, which can be employed in a proportion of 0.5 to 10% byweight relative to the sum of the components (A)+(C);

pulverulent inorganic fillers such as talc, silica or calcium carbonate,it being possible for these fillers to be employed in a proportion of 2to 40% by weight relative to the sum of the components (A)+(C).

The crosslinkable compositions according to the invention can be usedfor being deposited as a layer on the surface of a substrate and forbeing made to crosslink in order to protect the said substrate against,for example, oxidation and/or moisture, scratching, or else fordecorative purposes.

The crosslinking of the compositions according to the invention is adouble crosslinking:

1) by a radical route, of the copolymer (A) with itself or with thediluent (B) if the latter is reactive in the presence of thephotoinitiator or photoinitiating system (D); and

2) by reaction of the carboxylic acid groups of the copolymer (A) withthe epoxy groups of the component (C) in the presence of the catalyst(E).

The crosslinking 1) must be performed by irradiation.

The originality of the present invention is that, in order to ensure thecrosslinking 2), by virtue of the presence of the catalyst (D), it ispossible to carry on as far as doing without heating, with the resultthat there is no dissociation of the alkyl (meth)acrylate from thestyrene-carboxylic anhydride copolymer, or else heating conditions(temperature, duration) can be provided such that there is also nodissociation of the alkyl (meth)acrylate from the styrene-carboxylicanhydride copolymer, in contrast to the compositions according to U.S.Pat. No. 5,296,334 and EP-0 366 333. Since the hydroxyalkyl(meth)acrylate groups have not dissociated from the styrene-carboxylicanhydride copolymer, the latter therefore carries its carboxylic acidgroups as the only ones available for a reaction with themulti-functional epoxide. In these conditions a single polymer networkwill always be formed, ensuring the hardness sought after for theapplications of the invention.

Thus, a second subject of the present invention consists of a processfor coating substrates, consisting in applying onto a substrate a layerof the crosslinkable composition described above and then, whereappropriate, in subjecting the coated substrate to a stage of thermalcrosslinking and in subsequently subjecting it to a stage ofcrosslinking by irradiation.

A large number of porous and nonporous substrates may be mentioned assubstrates that can be subjected to the process of coating according tothe invention, such as especially:

metals like iron, steel, special steels, zinc, aluminium, and the like;

plastics and synthetic resins such as polyolefins, polycarbonate,polystyrene, polyethylene terephthalate, poly(vinyl chloride) andacrylonitrilebutadiene-styrene resins;

natural materials such as wood and rubber, and synthetic materials suchas melamine-formaldehyde resin laminates, agglomerates and the like.

Within the scope of the process according to the invention the layer ofcrosslinkable composition is applied in the form of a thin-to-semithickcoating ranging from a thickness of 2 μm to approximately 5 mm. Theapplication of the crosslinkable composition onto the substrate can beperformed by any known method such as a gun, roll, curtain and the like.

The optional thermal crosslinking stage is performed, for example, at atemperature of 80° to 150° C., preferably 90° to 120° C., for a periodof at least 15 seconds and in most cases not exceeding approximately 10minutes, this period being proportionately longer the lower thetemperature and the smaller the proportion of the catalyst (E) present.Thermal crosslinking can be performed, in a manner known per se, in aconvection oven or else in an infrared oven.

Finally, in the stage of crosslinking using irradiation, which is knownper se, any source of high-energy radiation can be employed, such as,for example, the electron beam or else ultraviolet radiation asobtained, for example, by means of a mercury lamp.

In the case where a crosslinking is conducted by providing a heatingstage preceding the irradiation stage, a crosslinked composition isobtained, characterized in that it includes a single network formedthroughout the thickness of the said coating by the partially esterifiedstyrene-carboxylic anhydride copolymer (A) which, in the network, hasbeen maintained without dissociation of its hydroxyalkyl (meth)acrylategroups and has had its carboxylic acid groups which have reacted withthe epoxy groups of the component (C) in the presence of the catalyst(E), the said copolymer (A) having also been subjected, in the presenceof the photoinitiator or of the photoinitiating system (D), tocrosslinking by a radical route with itself and/or with the diluent (B)if the latter is reactive.

In the case where no heating stage preceding the irradiation stage isenvisaged, a crosslinked composition is obtained in which, throughoutthe irradiated parts, the copolymer (A) has been subjectedpredominantly, in the presence of the photoinitiator or of thephotoinitiating system (D), to crosslinking by a radical route withitself and/or with the diluent (B) if the latter is reactive and, in theshadow regions, the copolymer (A) has been predominantly subjected, inthe presence of the catalyst (E), to crosslinking by reaction of itscarboxylic acid groups with the epoxy groups of the component (C). Thenetwork is here also a single network, since the copolymer (A) has notbeen dissociated either.

It can be seen therefore that the crosslinking is complete in all cases,with the result that it can be said that the invention makes it possiblein particular to solve the problem of the formation of protective and/ordecorative coatings on substrates exhibiting shadow regions, with theadditional advantage that it is possible not to heat the substrate. Itis therefore not necessary to have an oven available for heating thecoated substrate, and this may be an advantage from the viewpoint ofeconomics. A network will be obtained which will not be of the sametype, depending on whether the covered region is a shadow region or not(e.g. bottom of the pores in wood), but it is nevertheless possible tobe certain that the crosslinking will take place everywhere, without anyrisk of subsequent exudation of monomers in the case of porous wood.

Bearing in mind the substrates referred to above, the process accordingto the invention will advantageously find applications in the motorvehicle industry, for protective coating of components under the bonnet,and in the furniture and interior decoration industry, and in the fieldof floor coverings.

The examples below are provided by way of illustration and withoutlimiting the present invention. In these examples the parts andpercentages are given by weight. The parts are given for a total of 100parts and the percentages relate to these 100 parts. The compounds ofthe "SR" series used in these examples are marketed by Cray Valley, andthose of the "Sarbox" series by Sartomer.

EXAMPLE 1

A crosslinkable composition is prepared by mixing:

58.8 parts of a polyglycol diepoxide resin marketed by Dow Chemicalunder the reference "DER 732";

41.2 parts of a styrene-maleic anhydride resin modified by partialesterification with hydroxyethyl methacrylate;

27.5% of polyethylene glycol diacrylate diluent (Mw=400), theabovementioned styrene-maleic anhydride resin being marketed as amixture with the diluent under reference Sarbox® SB-500K60;

1.6% of tri(dimethylaminomethyl)phenol marketed by Cray Valley Ltd underthe reference Synolide® 968; and

3.3% of 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-1-butanonemarketed by Ciba-Geigy under the reference Irgacure® 369.

The crosslinkable composition thus obtained is then applied to thefollowing substrates: untreated steel, rubber, etc. by means of a gun,in the form of a coating approximately 25 μm in thickness, and is thensubjected to a stage of crosslinking at 100° C. for 2 minutes and,finally, to a stage of irradiation with ultraviolet radiation by meansof a mercury lamp, at a speed of 4 m/min and under a lamp power of 120watt/cm.

The adhesion of the crosslinked coating thus obtained to all thesubstrates used in the experiments is excellent.

EXAMPLE 2

A crosslinkable composition is prepared by mixing:

37.5 parts of 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane marketedby Union Carbide under the reference "UVR 6110";

62.5 parts of a styrene-maleic anhydride resin modified by partialesterification with hydroxyethyl methacrylate;

72% of a diluent made up of:

9.4% of triacrylate acidic adhesion promoter marketed under thereference "SR 9051"; and

62.6% of ethoxylated trimethylolpropane triacrylate marketed under thereference "SR 454", the abovementioned modified styrene-maleic anhydrideresin being marketed as a mixture with the diluent "SR454" under thereference Sarbox® SB-500E50;

6.2% of benzyltrimethylammonium chloride;

9% of 2-hydroxy-2-methyl-1-phenyl-1-propanone marketed by Ciba-Geigyunder the reference "Darocur D1173".

The COOH/epoxy ratio is 0.5.

The above composition is applied onto a glass substrate, by means of aroll, in the form of a coating of 36 μm thickness, and then subjected toa stage of crosslinking at 100° C. for 10 minutes and, finally, to astage of irradiation with ultraviolet radiation by means of a mercurylamp. The coating obtained was hard on leaving the machine.

The same composition was applied, by means of a roll, onto a woodsubstrate in which a succession of cavities of increasing depth, rangingup to 4 mm, had been made. The coating formed, filling the cavities, hada thickness of 36 μm on the surface of the substrate. The latter wassubjected to a stage of crosslinking at 100° C. for 10 minutes and thento a stage of irradiation with ultraviolet radiation by means of amercury lamp, at a speed of 8 m/min and under a lamp power of 120watt/cm. The coating obtained had a hard surface when it came out andwas polymerized to a depth of 4 mm.

The same composition was applied in the same conditions, subjected to anirradiation stage (8 m/min-120 watt/cm) and was then left at ambienttemperature for a few days. The coating obtained has a hard surface andis polymerized to a depth of 4 mm.

EXAMPLE 3

A crosslinkable composition is prepared by mixing:

50 parts of bisphenol A diglycidyl ether marketed by Dow Chemical underthe reference "DER 331";

50 parts of a styrene-maleic anhydride resin modified by partialesterification with hydroxyethyl methacrylate;

72.6% of a diluent made up of:

7.6% of triacrylate acidic adhesion promoter marketed by Cray Valleyunder the reference "SR 9051";

15% of 1,6-hexanediol diacrylate marketed under the reference "SR 238";and

50% of ethoxylated trimethylolpropane triacrylate reactive diluentmarketed under the reference "SR454", the abovementioned modifiedstyrene-maleic anhydride resin being marketed as a mixture with thediluent "SR454" under the reference "Sarbox® SB-500E50".

5% of benzyltrimethylammonium chloride;

7.6% of 2-hydroxy-2-methyl-1-phenyl-1-propanone marketed by Ciba-Geigyunder the reference "Darocur D1173".

The COOH/epoxy ratio is 0.5.

When applied in the same conditions as in Example 2, this crosslinkablecomposition gave the same results.

We claim:
 1. Crosslinkable compositions suitable for use as a radiationcurable coating for substrates with regions inaccessible to irradiationcomprising:(A) a modified copolymer which is a partial ester of astyrene-carboxylic anhydride copolymer and a hydroxyalkyl (meth)acrylate having carboxylic acid functional groups capable or reactingwith epoxy functional groups in the presence of a catalyst of thereaction between epoxy groups and carboxyl groups; (B) at least onediluent for the said modified copolymer; (C) at least onemultifunctional epoxide having epoxy functional groups capable ofreacting with said carboxylic acid functional groups in the presence ofa catalyst of the reaction between epoxy groups and carboxyl groups; (D)at least one photoinitiator or photointiating system capable ofinitiating photopolymerization and (E) at least one catalyst of thereaction between said carboxylic acid functional groups and epoxyfunctional groups, said catalyst present in a proportion of 1 to 10% byweight, relative to the sum of components (A) and (C).
 2. Crosslinkablecomposition according to claim 1, characterized in that saidstyrene-carboxylic anhydride copolymer has a number-average molecularweight of between 500 and 4000 and a molar ratio of styrene tocarboxylic anhydride of between 1 and
 4. 3. Crosslinkable compositionaccording to claim 1, further including a monovalent alcohol formodifying the styrene-carboxylic anhydride copolymer chosen fromaliphatic alcohols whose chain has from 1 to 22 carbon atoms. 4.Crosslinkable composition according to claim 1, characterized in thatthe multifunctional epoxide (C) is chosen from cycloaliphatic, aliphaticand aromatic diepoxidized resins, aliphatic triepoxidized resins andtetraepoxidized resins, and multi-epoxidized resins.
 5. Crosslinkablecomposition according to claim 1, characterized in that the molar ratioof the hydroxyalkyl (meth) acrylate to the carboxylic anhydridefunctional groups present in the styrene-carboxylic anhydride copolymerdoes not exceed
 1. 6. Crosslinkable composition according to claim 1,characterized in that the molar ratio of the carboxylic acid functionalgroups of the partially esterified copolymer to the epoxy functionalgroups of the multifunctional epoxide is between 0.3 and
 2. 7.Crosslinkable composition according to claim 1 characterized in that theproportion of diluent is between 5% and 200% by weight of the sum of thecomponents (A)+(C).
 8. Crosslinkable composition according to claim 1characterized in that the at least one photoinitiator or photoinitiatingsystem capable of initiating photopolymerization is present in aproportion of 2 to 15% of the weight of the sum of the components(A)+(C).
 9. Crosslinkable composition according to claim 1,characterized in that the catalyst (E) of the reaction between thecarboxylic acid functional groups of the modified copolymer (A) and theepoxy functional groups of the multifunctional epoxide (C) is selectedfrom the group consisting of tertiary amines, betaines, organic oniumacetates, triphenylphosphite chromium acetate and quaternary salts oftertiary amines.
 10. Crosslinkable composition according to claim 1,characterized in that it additionally includes at least one additivechosen from inorganic or organic pigments, polymer waxes and pulverulentinorganic fillers.
 11. Process for coating substrates, characterized inthat it consists in applying onto a substrate a layer of a crosslinkablecomposition as defined in claim 1; reacting said carboxylic acidfunctional groups with said epoxy groups; and crosslinking saidcomposition by irradiation.
 12. Process for coating substrates accordingto claim 11, including the thermal crosslinking stage which is performedat a temperature of 80° to 150° C. for a period of 15 seconds to 10minutes.
 13. Process according to claim 12, characterized in that thethermal crosslinking stage is performed at a temperature of 90 to 120°C.
 14. Substrate with regions inaccessible to irradiation comprising acoating obtained by crosslinking a composition as defined in claim 1,with crosslinking taking place everywhere.
 15. Coated substrateaccording to claim 14, characterized in that the network is homogeneousover the whole coating, the crosslinkable composition having beensubjected to a stage of heating followed by the irradiation stage. 16.Coated substrate comprising shadow regions or regions, inaccessible tothe irradiation, according to claim 14, characterized in that thecoating is made up of a crosslinked composition in which, throughout theirradiated parts, the copolymer (A) has been subjected, in the presenceof the photoinitiator or of the photoinitiating system (D), tocrosslinking by a radical route with itself and/or with the diluent (B)if the latter is reactive, and crosslinking by carboxylic acid-epoxidereaction and, in the shadow regions, the copolymer (A) has beenpredominantly subjected, in the presence of the catalyst (E), tocrosslinking by reaction of its carboxylic acid groups with the epoxygroups of the component (C), the crosslinkable composition having beensubjected only to an irradiation stage.
 17. Coated substrate accordingto claim 14, characterized in that it consists of a substrate which hasa surface with shadow regions.
 18. Coated substrate according to claim14, wherein said coating in the crosslinked state includes a networkformed by the partially esterified styrene-carboxylic anhydridecopolymer (A) which, in the network, has been maintained withoutdissociation of its hydroxyalkyl (meth) acrylate groups and has had itscarboxylic acid groups which have reacted with the epoxy groups of thecomponent (C) in the presence of the catalyst (E), the said copolymer(A) having also been subjected in the presence of the photoinitiator orof the photoinitiating system (D) to crosslinking by a radical routewith itself and/or with the diluent (B) if the latter is reactive, theepoxy groups of the component (C) having been totally or substantiallytotally consumed by reaction with the carboxylic acid groups of thecomponent (A).
 19. Process according to claim 11, where thecrosslinkable composition is only subjected to irradiation, withcrosslinking taking place everywhere on the coated substrate.
 20. Coatedsubstrate prepared by the process of claim 19.